1. Field of the Invention
The invention relates to a process for the catalytic preparation of alkali metal alkoxides from alkali metal amalgams and alcohols.
2. Description of the Background
Alkali metal alkoxides, particularly those whose alcohol component contains up to 4 carbon atoms, are valuable chemicals. They are used, for example, as catalysts in the synthesis of many organic compounds. This has given predominantly the alkoxides of sodium and potassium practical importance. A number of methods are known for preparing alkali metal alkoxides F. A. Dickes, Ber. Dtsch. Chem. Ges. 63, 2753 (1930)!. Thus, solutions of alkali metal hydroxides in an alcohol contain alkali metal alkoxide in equilibrium. Removal of the water present in this equilibrium, for example by distillation, gives pure alkali metal alkoxides. However, particularly in the case of low-boiling alcohols, a great deal of energy is required for this method of shifting the equilibrium.
Pure alkali metal alkoxides are obtained directly by dissolving an alkali metal in the corresponding alcohol. Thus, sodium and potassium react violently with lower aliphatic alcohols such as methanol and ethanol, giving off hydrogen. Higher alcohols such as propanols and butanols are preferably reacted with the alkali metals above the melting point of the latter, if desired under pressure and with stirring. The method of preparing the alkali metal alkoxides directly from metal and alcohol hardly comes into question for a commercial process, because the alkali metals required as starting materials are too expensive.
It is more economical to use, as a source of alkali metal, the liquid alkali metal amalgam formed in chloralkali electrolysis by the mercury method.
The reaction of alkali metal amalgam with alcohols and also the use of catalysts for this reaction are known.
R. B. MacMullin, Chemical Engineering Progress, September 1950, p. 440, mentions, inter alia, the reaction of alkali metal amalgam with methanol in a reactor containing graphite as catalyst. In U.S. Pat. No. 2,336,045 and 2,761,880, materials which are not amalgamated, for example, iron, graphite or mixtures thereof, are proposed as catalysts. U.S. Pat. No. 2,069,403 describes metal meshes comprising heavy metal alloys as catalysts.
The process according to the European Patent 0 177 768 enables the reaction between the amalgam and the alcohol to be accelerated significantly. For the reaction, use is made of a bed of particulate anthracite whose surface is coated with heavy metal oxide or a mixture of heavy metal oxides. Amalgam and alcohol are fed in continuously according to the countercurrent principle; the products are taken off continuously.
In this reaction, the combination of the oxides of nickel and molybdenum has a particularly high activity.
Preference is given to reacting aliphatic alcohols having from 1 to 4 carbon atoms. However, other alcohols can also be used as the starting material in this process, for example, aliphatic alcohols having more than 4 carbon atoms.
However, the prior art thus described is still unsatisfactory. For example, only from 60 to 70% of the sodium introduced in the amalgam are reacted with methanol. A second process step for removing the remaining alkali metal from the amalgam thus remains indispensable. In the case of the reaction of sodium amalgam with ethanol, the proportion which reacts is only from 40 to 50%. Moreover, the life of the oxide-coated catalyst is unsatisfactory. Over the course of about one year, the proportion which reacts drops so much that the catalyst has to be replaced.
It is therefore an object of the present invention to develop a process which avoids the above-mentioned disadvantages.